Causes, incidence, and risk factors

Lactose intolerance happens when the small intestine does not make enough of the enzyme lactase. Enzymes help the body absorb foods. Not having enough lactase is called lactase deficiency.

Babies' bodies make this enzyme so they can digest milk, including breast milk.

Premature babies sometimes have lactose intolerance. Children who were born at full term usually do not show signs of lactose intolerance until they are at least 3 years old.

Lactose intolerance can begin at different times in life. In Caucasians, it usually affects children older than age 5. In African Americans, lactose intolerance often occurs as early as age 2.

Lactose intolerance is more common in people with Asian, African, Native American, or Mediterranean ancestry than it is among northern and western Europeans.

Lactose intolerance is very common in adults and is not dangerous. Approximately 30 million American adults have some amount of lactose intolerance by age 20.

Causes of lactose intolerance include:

• Bowel surgery

• Infections in the small intestine from viruses or bacteria, which may damage the cells lining the intestine (most often in children)

• Intestinal diseases such as celiac sprue